For turning a stepped square shoulder on a workpiece, a cutting tool assembly requires a cutting insert with an acute operative insert cutting corner, a tool back clearance angle .kappa..sub.n along its inoperative cutting edge and an obtuse entering angle .kappa. along its operative cutting edge. Such an entering angle enables an outwardly directed feed out movement to square out a shoulder, in particular, an outwardly directed radial feed out movement in the case of external longitudinal turning operations and an outwardly directed axial feed out movement in the case of external radial turning operations.
In view of these restrictions, cutting inserts for turning stepped square shoulders are either rhomboidal or triangular, thereby having respectively two or three indexable insert cutting corners when single-sided. Such cutting inserts are, for example, as illustrated and described in EP 0 162 029 A2, each insert cutting corner being formed as a protruding nose portion at the junction between centrally depressed insert sides. The cutting inserts are preferably double sided so as to be respectively formed with four or six indexable insert cutting corners.